A bomb went off at a courthouse in Greece's second largest city Thessaloniki today, but there were no injuries and only minor damage, police officials said.
The blast came after a similar explosion outside the main prison in the capital Athens last night which slightly injured two people and damaged dozens of shops and homes.
An unidentified caller warned a Greek TV station and a newspaper that a bomb would explode in the Thessaloniki courthouse, police said.
"There was an explosion in the toilets of the main courthouse in Thessaloniki. There are no reports of injuries so far," a police official said. .
Another official said the bomb was placed in a plastic bag in the basement of the building.
Bomb attacks by militant groups are frequent in Greece and usually target police, public buildings or businesses.
In March, a 15-year-old boy was killed and his mother and sister wounded after a bomb exploded outside a building in central Athens, in the first deadly bomb attack in years.
Urban violence has increased after the police shooting of a teenager in December 2008, which prompted weeks of riots.
Social unrest is also picking up after Greece took belt-tightening measures, include wage cuts and tax hikes in recent months, aimed at pulling Greece out of a debt crisis.